Fluid process systems typically use valves such as, for example, rotary valves to control temperature, pressure, and other parameters associated with a fluid control process. Rotary valves typically have a valve stem or shaft that is mechanically coupled to an actuator. In operation, the actuator may rotate the valve shaft to cause a control element (e.g., a disc) to move between an open position that permits the passage of fluid through the valve and a closed position that substantially prevents the passage of fluid through the valve. Rotary valves are typically installed in-line with a pipe so that as the control element (e.g., a disc) moves (i.e., opens/closes), the flow of fluid through the valve and, thus, through the pipe may be varied (e.g., in a throttling control operation or an on/off operation).
As is known, actuators are typically coupled to a shaft of a valve to operate the valve between an open position and a closed position and may be implemented using electric, pneumatic, and/or hydraulic device(s). To facilitate the compatibility of process control valves with a variety of actuators, many available process control valves have shafts that are compliant with well-known standards. For example, the International Standards Organization (ISO) has developed a standard for square shafts that specifies shaft size, shaft dimensions, and shaft extension. Adherence to the ISO standard ensures that actuators and valves made by multiple manufacturers can be interchangeably coupled to each other without requiring modification of the actuators or valves. In particular, the valve shaft specification or ISO standard is particularly advantageous when purchasing off-the-shelf actuators.
Many off-the-shelf actuators provide shaft receptacles having a square bore that comply with the ISO standard. The square bore is typically manufactured using a broaching technique in which a thick saw-like cutting tool having a plurality of teeth is driven through a solid shaft or receptacle. In this manner, material is removed in a precise manner to form a bore dimensioned to receive a square valve shaft. However, broaching is an undesirable technique due to the precision or tolerances required to provide properly dimensioned bores (i.e., bores that are not too large or too small). In many instances, to ensure that the dimensions of the shaft receptacle are compliant with the ISO standard, the inner dimensions of the shaft receptacle are made substantially larger than the outer dimensions of a valve shaft.
For most on/off applications, the inner dimensions of the shaft receptacle may be significantly larger than the outer dimensions of the valve shaft without compromising operation. However, for throttling applications, in which the position of a control element (e.g., a disc) is varied (e.g., modulated about a control point) between a fully closed and a fully open position, oversized shaft receptacles are not suitable. An oversized shaft receptacle typically results in a loose mechanical coupling and, thus, lost motion between the shaft receptacle and the shaft of the process control device.
Lost motion may be generally defined as the difference in angular rotation between a shaft receptacle and a shaft and is typically a result of a loose coupling between the shaft receptacle and the shaft. For example, if a loose coupling is made between a shaft receptacle and a substantially square shaft, the angular rotation of the shaft receptacle may be different from the rotational displacement of the shaft.
In general, lost motion may lead to inaccurate positioning of the valve disc and poor control over the fluid flowing through the valve. Lost motion may be reduced by placing a collet in a lever in a valve assembly actuator. The collet couples the valve shaft to the lever, which is rotated by the actuator to open and close the valve. Collets provide a substantially tight coupling between the lever and the valve shaft without requiring the use of wedges, shaft keys, or the like.
When a valve assembly is disassembled, for example during an outage or for routine maintenance, the valve shaft must be removed from the actuator. Removing the valve shaft from the actuator may be accomplished by decoupling the collet from the lever. Typically, collets are removed with the use of blunt force such as, for example, by hitting the actuator with a hammer until the collet is unseated or loosened from the lever. This is undesirable as it requires a lot of effort (especially for large shaft sizes), and hammering on the end of the collet or valve shaft has the potential to cause internal damage to the valve assembly.